Feb 3, 2016
One morning (it was Friday, November 3), when Morgenthau came to the bedside tense with worry over some pressing problem and suggested that the [gold] price change that day be considerably greater than the 10 to 15 cents of immediately preceding days. Roosevelt promptly announced that the increase would be 21 cents. Why that figure? Because 'three times seven' is a lucky number, said Roosevelt, his face straight but his blue eyes twinkling at Morgenthau's recoil from such frivolous dealing with a serious matter.
Quoted by Scott Sumner in The Midas Paradox, pg. 268
Scott introduces this section by saying Monetary policy doesn’t get much more exogenous than this.